Carbon dioxide sensor

ABSTRACT

A carbon oxide sensor having high sensitivity capable of reduction of influence of humidity is provided. A detection electrode and a counter electrode are provided on one side of an electrolyte, respectively. The detection electrode contains a metal oxide and a metal carbonate including plural components. The metal carbonate preferably contains a first component of alkali metal carbonate and a second component of at least one element selected from the group consisting of alkali earth metal carbonate, transition metal carbonate, zinc carbonate, cadmium carbonate, indium carbonate, lead carbonate and bismuth carbonate. Specifically, it is preferable to contain the first component including Li 2 CO 3  and the second component including at least one element selected from the group consisting of BaCO 3 , CaCO 3  and SrCO 3 .

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a carbon dioxide sensor used foran indoor and outdoor environmental monitoring and control, anagro-industrial process such as greenhouse horticulture, disasterprevention, a measurement of metabolic function of body surface, and amedical care. 2. Description of the Related Art

[0003] In recent years, a need for carbon dioxide sensor is increasingparticularly in detection of dirty air in the room caused by the spreadof air conditioner, detection of contamination air in livestockfacilities, control of the growth of plants in greenhouse and variousindustrial processes, and various kinds of carbon dioxide sensors havebeen reported.

[0004] Specifically, an infrared absorption carbon dioxide sensor hasbeen in practical use, for example. However, this carbon dioxide sensoris not widely used because of the size and the cost of the apparatus. Inaddition, a carbon dioxide sensor using a semiconductor has been also inpractical use; however, it is difficult to monitor only the carbondioxide concentration due to poor carbon dioxide selectivity.

[0005] On the other hand, a carbon dioxide sensor using a solidelectrolyte has been proposed as a compact and inexpensive sensor. Anexample of such carbon dioxide sensor is so-called an electromotiveforce detection sensor obtained by forming a pair of electrodes on asolid electrolyte having an alkali metal ion conductivity such asNASICON (sodium super ion conductor: Na₃Zr₂Si₂PO₁₂). One electrode isused as a detection electrode with a metal carbonate layer such assodium carbonate which forms a dissociation equilibrium with carbondioxide. The other electrode is used as a carbon dioxide non-sensitiveelectrode (refer to Maruyama et al. “Solid State Ionics”, Vol 23, No.1/2, p.107-112 (1987)).

[0006] According to this carbon dioxide sensor, an excellent carbondioxide sensing property can be achieved. However, there has been aproblem that this carbon dioxide sensor is susceptible to humidity,because the metal carbonate is used as the detection electrode.Therefore, a carbon dioxide sensor that is lightly affected by humidityby utilizing the mixture of alkali metal carbonate and alkali earthmetal carbonate, or solid solution for the detection electrode has beenproposed (refer to Publication of Examined Application No. Hei 7-85071,Japanese Patent No. 2598172, Japanese Patent No. 2974088 and JapanesePatent No. 2974090, for example).

[0007] The operation temperature of the carbon dioxide sensor is as highas 400° C. to 700° C., so the overall power consumption of the sensor ishigh and there has been a problem that the heat deterioration ofmaterials occur. Further, heat of a few hundreds of temperature raisestemperature around the sensor even from a small heater and generatesconvection of the air. This affects subtly on the environment formonitoring, which is a problem.

[0008] Accordingly, a carbon dioxide sensor using a tin oxide (SnO₂)semiconductor containing stibium (Sb) or vanadium (V) as the detectionelectrode has been proposed (refer to S. Breikhin et al. “AppliedPhysics”, A57, 37-43 (1993), for example). Another carbon dioxide sensorin which the detection electrode is formed of a metal oxide layer, or ametal oxide layer dispersed metal carbonate or hydrogen carbonate hasbeen proposed (refer to Japanese Patent Laid-Open No. Hei 11-271270 orJapanese Patent Laid-Open No. 2000-88799). These carbon dioxide sensorsare operatable at room temperature.

[0009] However, the carbon dioxide sensor using the tin oxidesemiconductor as the detection electrode requires four or more minutesfor response and the difficulty of prompt measurement has been aproblem. In addition, there has been another problem that the responsetime or sensitivity varies by humidity because water vapor is involvedin the detection mechanism in this carbon dioxide sensor. Also, thecarbon dioxide sensor using the metal oxide is susceptible to humidity.Although the influence of humidity on the sensitivity is small, theabsolute value of the electromotive force output varies and thecorrection is required according to humidity, which is a problem.

SUMMARY OF THE INVENTION

[0010] The present invention has been achieved in view of the aboveproblems. It is an object of the invention to provide a carbon dioxidesensor which is operatable at room temperature and reduce the influenceof humidity.

[0011] A first carbon dioxide sensor of the invention is provided with adetection electrode and a counter electrode on an electrolyte, and thedetection electrode comprises a metal oxide layer including a metaloxide and a metal carbonate layer including metal carbonate placedbetween the metal oxide layer and the electrolyte.

[0012] In the first carbon dioxide sensor of the invention, the metaloxide layer and the metal carbonate layer are laminated, so it isoperative at room temperature and has less influence of humidity.

[0013] Preferably, the metal carbonate layer includes lithium carbonateand is formed at a temperature lower than a melting point of containingmetal carbonate.

[0014] The metal oxide layer preferably comprises at least one elementselected from the group consisting of tin oxide, indium oxide, cobaltoxide, tungsitc oxide, zinc oxide, lead oxide, copper oxide, iron oxide,nickel oxide, chromium oxide, cadmium oxide, bismuth oxide, manganeseoxide, yttrium oxide, antimony oxide, lanthanum oxide, cerium oxide,praseodymium oxide, neodymium oxide, silver oxide, lithium oxide, sodiumoxide, potassium oxide, rubidium oxide, magnesium oxide, calcium oxide,strontium oxide and barium oxide, and specifically, contains thecomposite oxide including tin and indium. The electrolyte includes ametal ion conductor, preferably.

[0015] A second carbon dioxide sensor of the invention is provided witha detection electrode and a counter electrode on an electrolyte, and thedetection electrode comprises a metal oxide and a metal carbonateincluding plural components.

[0016] In the second carbon dioxide sensor of the invention, thedetection electrode comprises the metal oxide and the metal carbonateincluding plural components, so it is operative at room temperature andhas less influence of humidity. Incidentally, the “metal carbonateincluding plural components” includes the cases where the plural kindsof metal carbonates are mixed without mutual chemical bonding, forms thecomplex carbonate with mutual chemical bonding, or both in thespecification.

[0017] The metal carbonate comprises a first component of alkali metalcarbonate and a second component of at least one element selected fromthe group consisting of alkali earth metal carbonate, transition metalcarbonate, zinc carbonate, cadmium carbonate, indium carbonate, leadcarbonate and bismuth carbonate, preferably. The first componentincludes lithium carbonate and the second component includes at leastone element selected from the group consisting of barium carbonate,calcium carbonate and strontium carbonate, preferably.

[0018] The detection electrode preferably comprises a metal oxide layerincluding metal oxide and a metal carbonate layer including metalcarbonate placed between the metal oxide layer and the electrolyte. Inthis case, the metal carbonate layer is formed at a temperature lowerthan a melting point of containing metal carbonate, preferably.

[0019] The metal oxide layer preferably comprises at least one elementselected from the group consisting of tin oxide, indium oxide, cobaltoxide, tungsitc oxide, zinc oxide, lead oxide, copper oxide, iron oxide,nickel oxide, chromium oxide, cadmium oxide, bismuth oxide, manganeseoxide, yttrium oxide, antimony oxide, lanthanum oxide, cerium oxide,praseodymium oxide, neodymium oxide, silver oxide, lithium oxide, sodiumoxide, potassium oxide, rubidium oxide, magnesium oxide, calcium oxide,strontium oxide and barium oxide, and specifically it is preferable tocontain the composite oxide including tin and indium.

[0020] The electrolyte preferably includes a metal ion conductor. Thedetection electrode and counter electrode are provided on the same sideof the electrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a sectional view showing a configuration of a carbondioxide sensor according to a first embodiment of the invention.

[0022]FIG. 2 is a sectional view showing a configuration of a carbondioxide sensor according to a second embodiment of the invention.

[0023]FIG. 3 is a sectional view showing a configuration of a carbondioxide sensor according to a third embodiment of the invention.

[0024]FIG. 4 is a sectional view showing a configuration of a carbondioxide sensor according to a variation of the third embodiment of theinvention.

[0025]FIG. 5 is a characteristic view showing the relation of the carbondioxide concentration and electromotive force by humidity according toExample 1 of the invention.

[0026]FIG. 6 is a characteristic view showing the relation of the carbondioxide concentration and electromotive force by humidity according toComparative Example 1 to Example 1 of the invention.

[0027]FIG. 7 is a characteristic view showing the relation of the carbondioxide concentration and electromotive force by humidity according toExample 2 of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Embodiments of the present invention will be described in detailbelow with reference to accompanying drawings. [First Embodiment]

[0029]FIG. 1 shows a configuration of a carbon dioxide sensor accordingto a first embodiment of the invention. In the carbon dioxide sensor, adetection electrode 10 is placed on the opposite side of a counterelectrode 20 sandwiching an electrolyte 30 in between. The detectionelectrode 10 and counter electrode 20 are connected to a potentiometerby leads 13 and 23. A quartz glass tube 40 is attached with an adhesivelayer 41 on the electrolyte 30 for preventing the counter electrode 20to be exposed in the measurement atmosphere.

[0030] The detection electrode 10 comprises a metal oxide layer 11including metal oxide and a metal carbonate layer 12 including metalcarbonate. The metal carbonate layer 12 is placed between the metaloxide layer 11 and the electrolyte 30. With such a layer structure, theinfluence of humidity can be decreased and the carbon dioxide sensingproperty can be improved. Preferably, the thicknesses of the metal oxidelayer 11 and the metal carbonate layer 12 are 10 nm to 500 μm, forexample, because higher effects can be achieved. The metal carbonate inthe specification means the so-called normal salt which does not containacid salt, that is, hydrogen carbonate and basic salt.

[0031] Preferably, the metal oxide layer 11 includes at least oneelement selected from the group consisting of tin oxide (SnO, SnO₂),indium oxide (In₂O₃), cobalt oxide (Co₃O₄), tungsitc oxide (WO₃), zincoxide (ZnO), lead oxide (PbO), copper oxide (CuO), iron oxide (Fe₂O₃,FeO), nickel oxide (NiO), chromium oxide (Cr₂O₃), cadmium oxide (CdO),bismuth oxide (Bi₂O₃), manganese oxide (MnO₂, Mn₂O₃), yttrium oxide(Y₂O₃), antimony oxide (Sb₂O₃), lanthanum oxide (La₂O₃), cerium oxide(CeO₂), praseodymium oxide (Pr₆O₁₁), neodymium oxide (Nd₂O₃), silveroxide (Ag₂O), lithium oxide (Li₂O), sodium oxide (Na₂O), potassium oxide(K₂O), rubidium oxide (Rb₂O), magnesium oxide (MgO), calcium oxide(CaO), strontium oxide (SrO) and barium oxide (BaO), for example.

[0032] Using these metal oxides allows prompt measurement at a lowtemperature. These metal oxides may be slightly deviated from thestoichiometric composition. When including two or more oxides, acomposite oxide or a mixture of these oxides may be included.

[0033] As the metal oxide, it is preferable to include at least oneelement selected from the group consisting of tin oxide, indium oxide,cobalt oxide, tungsitc oxide, zinc oxide, lead oxide, copper oxide, ironoxide, nickel oxide, chromium oxide, cadmium oxide and bismuth oxide,and further at least one element selected from the group consisting oftin oxide, indium oxide, lead oxide and tungsitc oxide because thehigher effects can be achieved. Also, when the composite oxide includingtin and indium is contained, the high conductivity is achieved, which ispreferable.

[0034] The metal carbonate layer 12 preferably includes at least oneelement selected from the group consisting of lithium carbonate(Li₂CO₃), sodium carbonate (Na₂CO₃), potassium carbonate (K₂CO₃),rubidium carbonate (Rb₂CO₃), cesium carbonate (Cs₂CO₃), magnesiumcarbonate (MgCO₃), calcium carbonate (CaCO₃), strontium carbonate(SrCO₃), barium carbonate (BaCO₃), manganese carbonate (Mn(CO₃)₂,Mn₂(CO₃)₃), iron carbonate (Fe₂(CO₃)₃, FeCO₃), nickel carbonate (NiCO₃),copper carbonate (CuCO₃), cobalt carbonate (Co₂(CO₃)₃), chromiumcarbonate (Cr₂(CO₃)₃), zinc carbonate (ZnCO₃), silver carbonate(Ag₂CO₃), cadmium carbonate (CdCO₃), indium carbonate (In₂(CO₃)₃),yttrium carbonate (Y₂(CO₃)₃), lead carbonate (PbCO₃), bismuth carbonate(Bi₂(CO₃)₃), lanthanum carbonate (La₂(CO₃)₃), cerium carbonate(Ce(CO₃)₃), praseodymium carbonate (Pr₆(CO₃)O₁₁) and neodymium carbonate(Nd₂(CO₃)₃), for example.

[0035] Using these metal carbonates allows the improvement of carbondioxide sensing property. These metal carbonates may be slightlydeviated from the stoichiometric composition. When including two or moremetal carbonates, a compound or a mixture of these carbonates may beincluded. As the metal carbonate, it is preferable to include alkalimetal carbonate, specifically, lithium carbonate because it can reducethe influence of humidity.

[0036] The metal carbonate layer 12 may further include metal hydrogencarbonate. This is because the metal hydrogen carbonate may be generatedduring the measurement of carbon dioxide.

[0037] Although a method of forming the metal oxide layer 11 and themetal carbonate layer 12 is not limited, a method that a powder paste ofthe metal carbonate is applied to the electrolyte 30 and heat-treated toform the metal carbonate layer 12, and then a metal oxide paste isapplied thereon and heat-treated to form the metal oxide layer 11 ispreferable.

[0038] In addition, the metal carbonate layer 12 and the metal oxidelayer 11 are preferably formed by a method that after applying thepowder paste of the metal carbonate to the electrolyte 30, the metaloxide is applied thereon and heat-treated at a temperature lower than amelting point of containing metal carbonate. If the metal carbonatelayer 12 and the metal oxide layer 11 are heated at a temperature higherthan the melting point of the metal carbonate, the metal carbonatereacts with the metal oxide or the electrolyte 30 and produce reactantand it is easily affected by humidity.

[0039] The preferable average grain size of the used metal oxide and themetal carbonated is 10 nm to 100 pm. A solvent not reacting with themetal oxide and the metal carbonate and having a relatively low roomsteam pressure and good workability is used as the solvent of the paste.In particular, α-terpineol, ethylene glycol, glycerin or the like ispreferable. The slurry viscosity is 0.01 Pa.s to 10,000 Pa.s,preferably.

[0040] The detection electrode 10 may have collector (not shown) on thetop surface or in the metal oxide layer 11. It is effective when theconductivity of the metal oxide layer 11 is not so high. However, whenusing the composite oxide including indium and tin, high conductivitycan be achieved, so no collector is required. The collector is formed ofmetal, preferably, any one or more elements of gold (Au), platinum (Pt),silver (Ag), rubidium (Rb), rhodium (Rh), palladium (Pd), iridium (Ir),nickel (Ni), copper (Cu) and chromium (Cr). The collector is preferablyformed of a porous metal to diffuse carbon dioxide. The porous metal ispreferably a metal mesh or a powder electrode which is formed bypressure bonding or screen printing a metal powder paste. In particular,a powder electrode is preferable.

[0041] The screen printing is a method of applying a metal powder pasteto a substrate through mesh screen and in this case a porous electrodein which metal particles are interconnected is formed. The average grainsize of the used metal powder is 10 nm to 100 μm and preferably 10 nm to10 μm for good printing. A solvent not reacting with the used metal andhaving a relatively low room steam pressure and good workability is usedas the solvent of the paste. In particular, α-terpineol, ethyleneglycol, glycerin or the like is preferable. The slurry viscosity is 0.01Pa.s to 10,000 Pa.s, preferably.

[0042] The porous metal formed by sputtering also can be used ascollector. Argon (Ar), helium (He), oxygen (O₂), nitrogen (N₂) and thelike are preferably used as a sputtering gas and the preferable pressureduring the deposition is within the range of 0.013 Pa to 66.7 Pa. Thesurface of the electrode can be porous by resistance heating deposition.

[0043] The counter electrode 20 has a thickness of 0.1 μm to 100 μm, forexample and formed of metal or metal oxide. Preferably, any one or moremetals constituting the collector (not shown) of the detection electrode10 or oxides of those metals, or the metal oxides constituting the metaloxide layer 11 is used for forming the counter electrode 20, forexample. Specifically, in the counter electrode 20 formed of the metaloxide, the influence of concurrent gas is reduced, higher carbon dioxideselectivity can be achieved, humidity resistance is improved, and theinfluence of humidity during the measurement under a low temperature isreduced in particular.

[0044] The counter electrode 20 is preferably formed of the porous metalor the porous metal oxide like the collector of the detection electrode10. Specifically, the powder electrode which is formed by pressurebonding or screen printing of the metal oxide powder paste ispreferable. The carbon dioxide sensor comprises a powder electrode 21and a gold mesh 22. The metal powder or the metal oxide powder forforming the powder electrode 21 preferably has an average grain size of10 nm to 100 μm, and more preferably 0.5 μm to 100 μm. A solvent notreacting with the used metals or metal oxides and having a relativelylow room steam pressure and good workability is preferably used as thesolvent of the paste. In particular, α-terpineol, ethylene glycol,glycerin or the like is preferable. The slurry viscosity is 0.01 Pa.s to10,000 Pa.s, preferably.

[0045] The electrolyte 30 has a thickness of 1 μm to 5 mm, for exampleand preferably include a metal ion conductor. Examples of the metal ionconductor are Na-β″ alumina, Na-β alumina, Na₃Zr₂PSi₂O₁₂, Na₃Zr₂Si₂PO₁₂(NASICON), Na-βGa₂O₃, Na—Fe₂O₃, Na₃Zr₂PSi₂P₂O₁₂, Li-β alumina,Li₁₄Zn(GeO₄)₄, Li₃Zn_(0.5)GeO₄, Li_(3.5)Zn_(0.25)GeO₄(LISICON), lithiumion exchange NASICON, Li₅AlO₄, Li_(1.4)Ti_(1.6)In_(0.4)P₃O₁₂, K-βalumina, K_(1.6)Al_(0.8)Ti_(7.2)O₁₆, K₂MgTi₇O₁₆, CaS or the like. Ofthese metal ion conductors, sodium ion conductor or lithium ionconductor is preferable and NASION, LISICON, lithium ion exchangeNASICON or the like is more preferable because the ion conductionrequired for the response of the sensor at a low temperature isconfirmed. These metal ion conductors may be slightly deviated from thestoichiometric composition. Polyelectrolyte can be used as well.

[0046] The electrolyte 30 may contain aluminum oxide (Al₂O₃), siliconoxide (SiO₂), zirconium oxide (ZeO₂), silicon carbide (SiC), siliconnitride (Si₃N₄), iron oxide (Fe2O₃) or the like with 50% by mass or lessin addition to the metal ion conductor as a reinforcer not to preventthe ion conductivity. These elements may be slightly deviated from thestoichiometric composition.

[0047] In order to form the electrolyte 30, any generally usedsolid-phase method, sol-gel method, coprecipitation method and the likecan be used, and preferably sol-gel method is used.

[0048] In the carbon dioxide sensor, when the detection electrode 10 isexposed to the measurement atmosphere, carbon dioxide in the measurementatmosphere diffuses into the metal oxide layer 11 and reaches the metalcarbonate layer 12. The dissociation equilibrium in the metal carbonateand carbon dioxide changes in the metal carbonate layer 12 and the metalion activity in the electrolyte 30 near the detection electrode 10 alsochanges. Thereby, the electromotive force is generated between thedetection electrode 10 and the counter electrode 20 and carbon dioxideconcentration is measured.

[0049] In particular, the carbon dioxide sensor has the layer structureof the metal oxide layer 11 and the metal carbonate layer 12 andtherefore, it is not susceptible to humidity and a stable measurementresults are achieved at room temperature at 30% or more humidity. Thecarbon dioxide sensing property is also improved. When lithium carbonateis included in the metal carbonate layer 12, or the metal carbonatelayer 12 is formed at a temperature lower than the melting point ofcontaining metal carbonate, more stable results can be achieved.

[0050] According to the embodiment, the carbon dioxide sensor comprisesthe metal carbonate layer 12 and the metal oxide layer 11 on theelectrolyte 30 in this order, so it is operatable at room temperatureand has less influence of humidity, and the carbon dioxide sensingproperty is improved. As a result, the carbon dioxide concentration canbe detected easily with high accuracy.

[0051] Specifically, when lithium carbonate is included in the metalcarbonate layer 12 or the carbonate layer 12 is formed at a temperaturelower than the melting point of containing metal carbonate, theinfluence of humidity can be decreased and the carbon dioxide sensingproperty is improved.

[0052] [Second Embodiment]

[0053]FIG. 2 shows the configuration of a carbon dioxide sensoraccording to a second embodiment of the invention. In the carbon dioxidesensor, a detection electrode 50 and a counter electrode 60 are placedon the same side of an electrolyte 70. The detection electrode 50 and acounter electrode 60 are connected to a potentiometer by leads. Thedetection electrode 50 may be placed on the opposite side of the counterelectrode 60 sandwiching the electrolyte 70 in between, as shown in FIG.1; however, with this configuration, that is, the detection electrode 50and the counter electrode 60 are placed on the same surface of theelectrolyte 70, it facilitate the extraction of the leads and themanufacturing process. In addition, the miniaturization of device can beimproved, which is preferable.

[0054] The detection electrode 50 has a thickness of, for example 0.1 μmto 100 μm and contains the metal oxide and the metal carbonate includingplural components. The metal oxide and the metal carbonate are dispersedwith each other.

[0055] The same metal oxides constituting the detection electrode 10 inthe first embodiment can be used, for example.

[0056] The metal carbonate contains a first component consisting of atleast one element of alkali metal carbonate and a second componentconsisting of at least one element of alkali earth metal carbonate andtransition metal carbonate, for example. The influence of humidity isreduced with such a mixture. Alkali metal carbonate, alkali earth metalcarbonate and transition metal carbonate may be mixed without mutualchemical bonding, mixed with mutual chemical bonding to form the complexcarbonate, or some of them form the complex carbonate and some of themare mixed. Each of alkali metal carbonate, alkali earth metal carbonateand transition metal carbonate may be mixed, bonded or both whenincluding plural kinds of components.

[0057] Alkali metal carbonate includes lithium carbonate (Li₂CO₃),sodium carbonate (Na₂CO₃), potassium carbonate (K₂CO₃), rubidiumcarbonate (Rb₂CO₃), or cesium carbonate (Cs₂CO₃), for example. Alkaliearth metal carbonate includes magnesium carbonate (MgCO₃), calciumcarbonate (CaCO₃), strontium carbonate (SrCO₃), or barium carbonate(BaCO₃), for example.

[0058] Transition metal carbonate is elements belong to Group III toGroup XI of long period periodic table. Examples of transition metalcarbonate are manganese carbonate (Mn(CO₃)₂, Mn₂(CO₃)₃), iron carbonate(Fe₂(CO₃)₃, FeCO₃), nickel carbonate (NiCO₃), copper carbonate (CuCO₃),cobalt carbonate (Co₂(CO₃)₃), chromium carbonate (Cr₂(CO₃)₃), silvercarbonate (Ag₂CO₃), yttrium carbonate (Y₂(CO₃)₃), lanthanum carbonate(La₂(CO₃)₃), cerium carbonate (Ce(CO₃)₃), praseodymium carbonate(Pr₆(CO₃)O₁₁) and neodymium carbonate (Nd₂(CO₃)₃). These metalcarbonates may be slightly deviated from the stoichiometric composition.

[0059] Preferably the first component includes at least lithiumcarbonate and the second component includes at least one elementselected from the group consisting of barium carbonate, calciumcarbonate and strontium carbonate because higher effects can beachieved. Specifically, it is preferable to include lithium carbonate inconcurrent with at least one element selected from the group consistingof barium carbonate, calcium carbonate and strontium carbonate.

[0060] The second component may be include at least one element selectedfrom the group consisting of zinc carbonate (ZnCO₃), cadmium carbonate(CdCO₃), indium carbonate (In₂(CO₃)₃), lead carbonate (PbCO₃) andbismuth carbonate (Bi₂(CO₃)₃). With these elements, the influence ofhumidity is reduced similarly. These elements may be included togetherwith the elements of alkali earth metal carbonate or transition metalcarbonate and be substituted for these elements. When including two ormore kinds of these elements, these elements and others may be mixed,bonded or both as described above.

[0061] The detection electrode 50 may contain metal hydrogen carbonate,because the metal hydrogen carbonate may be generated during themeasurement of carbon dioxide.

[0062] Although a method of forming the detection electrode 50 is notlimited, a method that a paste including the metal oxide powder and themetal carbonate powder is applied to the electrolyte 70 and heat-treatedat a temperature lower than a melting point of containing metalcarbonate, as described in the first embodiment, for example. If themetal oxide and the metal carbonate are heat-treated at a temperaturehigher than the melting point of the metal carbonate, the metalcarbonate reacts with the metal oxide or the electrolyte 70 and producereactant and it becomes susceptible to humidity.

[0063] The average grain size of the used metal oxide and the metalcarbonate, the solvent of the paste or the like are the same asdescribed in the first embodiment.

[0064] The detection electrode 50 may comprise the collector on the topsurface thereof or therein like the first embodiment. The structure ofthe collector is the same as the first embodiment.

[0065] The counter electrode 60 has a thickness of, for example about0.1 μm to 100 μm and comprises a standard layer 61 provided on theelectrolyte 70 and a protective layer 62 provided to cover the standardlayer 61.

[0066] The standard layer 61 is formed of metal or metal oxide, forexample. The metal or the metal oxide of the standard layer 61 ispreferably any one or more elements of the metal oxides of the detectionelectrode 10 in the first embodiment, the metal of the collector of thedetection electrode 10 or the oxides thereof, for example. When usingthe metal oxide for forming the standard layer 61, the influence ofconcurrent gas is reduced, the high carbon dioxide selectivity isachieved, the humidity resistance is improved, and the influence ofhumidity during the measurement under a low temperature is reduced inparticular.

[0067] The standard layer 61 is preferably formed of the porous metal orthe porous metal oxide and the powder electrode formed by pressurebonding or screen printing of the metal oxide powder paste is morepreferable. The details of the powder electrode are as described in thecounter electrode 20 in the first embodiment.

[0068] The protective layer 62 is provided for reducing the influence ofhumidity by preventing the standard layer 61 from exposing to themeasurement atmosphere and formed of fluororesin, inorganic ceramics,cobaltate or the like, preferably. The protective layer 62 isdispensable; however it is preferable to provide to reduce the influenceof humidity.

[0069] The electrolyte 70 is the same as the electrolyte 30 of the firstembodiment.

[0070] In the carbon dioxide sensor, when the detection electrode 50 isexposed in the measurement atmosphere, carbon dioxide in the measurementatmosphere diffuses into the detection electrode 50 and the dissociationequilibrium of the metal carbonate with carbon dioxide changes.Accordingly, the metal ion activity in the electrolyte 70 near thedetection electrode 50 changes. As a result, the electromotive force isgenerated between the detection electrode 50 and the counter electrode60 and carbon dioxide concentration is measured.

[0071] The carbon dioxide sensor contains the metal oxide and the metalcarbonate including plural components, so it is operative at roomtemperature and is not susceptible to humidity and the stablemeasurement results are achieved at room temperature at 30% or more ofhumidity. When including the first component of alkali metal carbonateand the second component of at least one element selected from the groupconsisting of alkali earth metal carbonate and transition metalcarbonate, more stable results can be achieved.

[0072] According to the embodiment, the detection electrode 50 containsthe metal carbonate including plural components, so it is operative atroom temperature and has less influence of humidity. As a result, thecarbon dioxide concentration can be detected easily with high accuracy.

[0073] Specifically, when including the first component of alkali metalcarbonate and the second component of at least one element selected fromthe group consisting of alkali earth metal carbonate, transition metalcarbonate, zinc carbonate, cadmium carbonate, indium carbonate, leadcarbonate and bismuth carbonate, and further including lithium carbonatein the first component and including at least one element selected fromthe group consisting of barium carbonate, potassium carbonate andstrontium carbonate in the second component, the influence of humiditycan be reduced more.

[0074] [Third Embodiment]

[0075]FIG. 3 shows the configuration of a carbon dioxide sensoraccording to a third embodiment of the invention. The carbon dioxidesensor has the same configuration as that of the second embodimentexcept that a detection electrode 80 has the layer structure as thefirst embodiment. Therefore, the same components are indicated by thesame numerals and the detail explanation thereof will be omitted.

[0076] The detection electrode 80 comprises a metal oxide layer 81including metal oxide, a metal carbonate layer 82 including pluralcomponents of metal carbonate. The metal carbonate layer 82 is providedbetween the metal oxide layer 80 and the electrolyte 70. With such alayer structure, the influence of humidity can be decreased and thecarbon dioxide sensing property can be improved. Preferably, thethickness of the metal oxide layer 81 is 10 nm to 500 μm and thethickness of the metal carbonate layer 82 is 10 nm to 500 μm, forexample, because higher effects can be achieved.

[0077] The metal oxide included in the metal oxide layer 81 and theplural components of the metal carbonate included in the metal carbonatelayer 82 are the same as that of the second embodiment. The metalcarbonate layer 82 may contain the metal hydrogen carbonate because themetal hydrogen carbonate may be generated during the measurement ofcarbon dioxide.

[0078] The metal oxide layer 81 and the metal carbonate layer 82 areformed similar to the first embodiment.

[0079] According to the embodiment, the metal carbonate layer 82 and themetal oxide layer 81 on the electrolyte 70 are comprised in this order,so the influence of humidity can be reduced and the carbon dioxidesensing property can be improved.

[0080] Specifically, when the metal carbonate layer 82 is formed at atemperature lower than the melting point of the used metal, the highereffects can be achieved.

[0081] In FIG. 3, the metal oxide layer 81 covers the top and sidesurfaces of the metal carbonate layer 82 completely and part of themetal oxide layer 81 contacts to the electrolyte 70. However, as shownin FIG. 4, the metal oxide layer 81 may cover at least part of the topsurface of the metal carbonate layer 82 and be not in contact with theelectrolyte 70.

EXAMPLE

[0082] Further, the specific examples of the invention will be describedhereinbelow.

Example 1

[0083] A carbon dioxide sensor as shown in FIG. 1 was formed. First,NASICON powder was prepared by sol-gel method and a discoid electrolyte30 having a diameter of 9 mm and a thickness of 1.2 mm was formed byusing the NASICON powder. Next, a gold paste was applied to the entiresurface of the electrolyte 30 together with a gold mesh 22 and a goldwire to be a lead 23, and a powder electrode 21 was formed by heattreatment at 800° C. for two hours in the atmosphere to form a counterelectrode 20. Subsequently, a quartz glass tube 40 of 9 mm in diameterwas attached to the electrolyte 30 on the side where the counterelectrode 20 was provided with an adhesive layer 41 using an inorganicadhesive.

[0084] After that, lithium carbonate powder and an organic solventcontaining 5% by mass of ethyl cellulose and 95% by mass of α-terpineolwere mixed with substantially equal weight each to form the paste, andthe paste was applied to the surface of the electrolyte 30.

[0085] Indium chloride (InCl₃) powder and tin chloride (SnCl₄) powderwere mixed by using water and heated at 1200° C. for two hours. Thecomposite oxide powder of indium and tin with a grain size of about 0.5μm to 1 μm was obtained. Then, the composite oxide powder and theorganic solvent containing 5% by mass of ethyl cellulose and 95% by massof α-terpineol were mixed with substantially equal weight each to formthe paste, and the paste was applied on the lithium carbonate pastelayer.

[0086] Subsequently, heat-treatment was performed at 500° C. which waslower than the melting point of lithium carbonate in the atmosphere for30 minutes to form a detection electrode 10 laminated a metal carbonatelayer 12 having a thickness of about 20 μm and a metal oxide layer 11having a thickness of about 20 μm. Thereby, the carbon dioxide sensorshown in FIG. 1 was obtained.

[0087] A carbon dioxide sensor as Comparative Example 1 to Example 1 wasprepared like Example 1 except that lithium carbonate powder and thecomposite oxide powder of indium and tin were mixed with the organicsolvent having substantially the equal weight with these powders to formthe paste and the paste was applied to the surface of the electrolyte30, and then the heat treatment was performed at 500° C. for 30 minutesin the atmosphere to form the detection electrode with about 40 μm inthickness.

[0088] The relation of electromotive force and carbon dioxideconcentration at room temperature at 30%, 50% and 70% humidity for theobtained carbon dioxide sensors of Example 1 and Comparative Example 1were examined. The results of Example 1 and Comparative Example 1 areshown in FIG. 5 and FIG. 6, respectively. The value n is calculated fromthe gradient showing the relation between the carbon dioxideconcentration and the absolute value of electromotive force and meansthe number of reaction electron regarding electrochemical reduction percarbon dioxide molecule. As shown in FIGS. 5 and 6, the difference ofthe absolute value of electromotive force to the carbon dioxideconcentration by humidity is reduced and the influence on thesensitivity is also reduced. Namely, when the metal carbonate layer 12and the metal oxide layer 11 are laminated in order from the electrolyte30 side, we have found that the carbon dioxide is operatable at roomtemperature and has less influence of humidity, and the carbon dioxidesensing property can be improved.

[0089] As seen in the comparison of FIGS. 5 and 6, the absolute value ofelectromotive force to the carbon dioxide concentration of Example 1 waslarger than that of Comparative Example 1. As a result, we have foundthat the laminate structure having the metal oxide layer 11 and themetal carbonate layer 12 improves the carbon dioxide sensing property.

Example 2

[0090] A carbon dioxide sensor was formed like Example 1 except thatlithium carbonate paste was applied to the electrolyte 30, heated at750° C., which was higher than the melting point of lithium carbonate,in the atmosphere, extracted at the time of melting and fusion bondingthe metal carbonate layer 12, and after that the composite oxide pasteof indium and tin was applied thereon and heated at 500° C. for 30minutes in the atmosphere to form the metal oxide layer 11.

[0091] The relation of electromotive force and carbon dioxideconcentration at room temperature at 30%, 50% and 70% humidity for theobtained carbon dioxide sensor of Example 2 were examined like the firstembodiment. The results of Example 2 are shown in FIG. 7. As seen in thecomparison of FIGS. 5 and 7, the difference of the absolute value ofelectromotive force to the carbon dioxide concentration by humidity ofExample 1 was smaller than that of Example 2. That is, when the metalcarbonate layer 12 is formed at a temperature lower than the meltingpoint of the metal carbonate, we have found that the carbon dioxidesensor is operatable at room temperature and has less influence ofhumidity, and the carbon dioxide sensing property can be improved.

[0092] A carbon dioxide sensor as Comparative Example to Example 2, thatis, lithium carbonate powder and the composite oxide powder of indiumand tin were mixed and fusion bonded at a temperature higher than themelting point of lithium carbonate to form the detection electrode isnot mentioned. However, because of the reaction of lithium carbonatewith the composite oxide or the electrolyte, it is supposed. that theproperties deserving evaluation cannot be obtained.

Examples 3-1 to 3-4

[0093] The carbon dioxide sensors shown in FIG. 1 were formed likeExample 1 except for the change of the configuration of the detectionelectrode 10. Specifically, the complex carbonate paste of alkali metalcarbonate of a first component and barium carbonate of a secondcomponent or alkali metal carbonate paste was applied to the electrolyte30 and then dissolved at a temperature higher than the melting point ofalkali metal carbonate in the atmosphere to fusion bond the metalcarbonate layer 12, and the composite oxide paste of indium and tin wasapplied thereon and heated at 500° C., which is lower than the meltingpoint of alkali metal carbonate in the atmosphere for 30 minutes to formthe metal oxide layer 11.

[0094] At this time, the complex carbonate of potassium carbonate andbarium carbonate was used in Example 3-1, potassium carbonate was usedin Example 3-2, the complex carbonate of lithium carbonate and bariumcarbonate was used in Example 3-3, and lithium carbonate was used inExample 3-4. Example 3-4 is the same as Example 2. The complex carbonatewas prepared by mixing alkali metal carbonate powder of the firstcomponent and barium carbonate powder of the second component at alkalimetal carbonate:barium carbonate=1:2 in molar ratio and melting themixed powder at a temperature higher than the melting point of alkalimetal carbonate.

[0095] The relation of electromotive force and carbon dioxideconcentration at room temperature at 30%, 50% and 70% humidity for theobtained carbon dioxide sensors of Example 3-1 to 3-4 were examined andthe sensitivity and variation of electromotive force at carbon dioxideconcentration of 350 ppm at 30% and 70% humidity were determined. Theresults are shown in Table 1. The sensitivity indicates the number ofreaction electron n as explained in Example 1. TABLE 1 Variation ofSensitivity electromotive force Metal carbonate (Number of reactionelectron n) at 350 ppm CO₂ (Fusion bonding layer) +% RH 50% RH 70% RH(30%→70% RH) Example 3-1 K₂CO₃—BaCo₃ 2.3 2.3 2.3 19 mV Example 3-2 K₂CO₃2.4 2.5 2.6 21 mV Example 3-3 Li₂CO₃—BaCO₃ 1.5 1.6 1.6 20 mV Example 3-4Li₂CO₃ 1.6 1.5 1.5 27 mV

[0096] As shown in Table 1, according to Examples 3-1 and 3-3 includingpotassium carbonate or lithium carbonate and barium carbonate, comparedto Examples 3-2 and 3-4 including only potassium carbonate or lithiumcarbonate, the variation of electromotive force by humidity was small.Namely, when the carbon dioxide sensor comprises the metal oxide layer11 and the metal carbonate layer 12 including the plural components, wehave found that it is operatable at room temperature and has lessinfluence of humidity.

Examples 4-1 to 4-6

[0097] The carbon dioxide sensors were formed like the Examples 3-1 to3-4 except that after applying the complex carbonate paste of alkalimetal carbonate and barium carbonate or alkali metal carbonate paste tothe electrolyte 30, the composite oxide paste of indium and tin wereapplied thereon and heated at 500° C. for 30 minutes to form the metaloxide layer 11 and metal carbonate layer 12. At this time, the complexcarbonate of sodium carbonate and barium carbonate was used in Example4-1, sodium carbonate was used in Example 4-2, the complex carbonate ofpotassium carbonate and barium carbonate was used in Example 4-3,potassium carbonate was used in Example 4-4, the complex carbonate oflithium carbonate and barium carbonate was used in Example 4-5 andlithium carbonate was used in Example 4-6.

[0098] The relation of electromotive force and carbon dioxideconcentration at room temperature at 30%, 50% and 70% humidity for thecarbon dioxide sensors of Examples 4-1 to 4-6 were examined likeExamples 3-1 to 3-4. The results are shown in Table 2. TABLE 2 Variationof Metal carbonate Sensitivity electromotive force (Non-fusion bonding(Number of reaction electron n) at 350 ppm CO₂ layer) 30% RH 50% RH 70%RH (30%→70% RH) Example 4-1 Na₂CO₃—BaCO₃ 1.7 1.6 1.6 12 mV Example 4-2Na₂CO₃ 1.6 1.6 1.7 21 mV Example 4-3 K₂CO₃—BaCO₃ 2.2 2.2 2.2 17 mVExample 4-4 K₂CO₃ 2.1 2.1 2.2 25 mV Example 4-5 Li₂CO₃—BaCO₃ 1.7 1.7 1.7 1 mV Example 4-6 Li₂CO₃ 1.7 1.8 1.8  7 mV

[0099] As shown in Table 2, similar to Examples 3-1 and 3-3, Examples4-1, 4-3, 4-5 including alkali metal carbonate and barium carbonate havesmaller variation of electromotive force by humidity. Namely, when thecarbon dioxide sensor comprises the metal carbonate layer 12 includingthe plural components, we have found that it is operatable at roomtemperature and has less influence of humidity.

[0100] Comparing Table 1 with Table 2, the variation of electromotiveforce by humidity of Examples 4-3 and 4-5 was smaller than that ofExamples 3-1 and 3-3. As a result, we have found that forming the metalcarbonate layer 12 at a temperature lower than the melting point of usedmetal carbonate can reduce the influence of humidity.

[0101] Additionally, from the results of Table 2, in Example 4-5 usinglithium carbonate as the first component, the variation of electromotiveforce could be as low as 1 mV. That is, when including lithiumcarbonate, the influence of humidity can be reduced.

Examples 5-1 to 5-6, 6-1 to 6-6

[0102] The carbon dioxide sensors shown in FIG. 3 were formed by fusionbonding the metal carbonate layer 82 of complex carbonate of alkalimetal carbonate and barium carbonate or of alkali metal carbonate likeExamples 3-1 to 3-4 as Examples 5-1 to 5-6. At this time, the complexcarbonate of sodium carbonate and barium carbonate was used in Example5-1, sodium carbonate was used in Example 5-2, the complex carbonate ofpotassium carbonate and barium carbonate was used in Example 5-3,potassium carbonate was used in Example 5-4, the complex carbonate oflithium carbonate and barium carbonate was used in Example 5-5 andlithium carbonate was used in Example 5-6.

[0103] The carbon dioxide sensors shown in FIG. 3 were formed by formingthe metal carbonate layer 82 of complex carbonate of alkali metalcarbonate and barium carbonate or of alkali metal carbonate at atemperature lower than the melting point of alkali metal carbonate likeExamples 4-1 to 4-6 as Examples 6-1 to 6-6. At this time, the complexcarbonate of sodium carbonate and barium carbonate was used in Example6-1, sodium carbonate was used in Example 6-2, the complex carbonate ofpotassium carbonate and barium carbonate was used in Example 6-3,potassium carbonate was used in Example 6-4, the complex carbonate oflithium carbonate and barium carbonate was used in Example 6-5 andlithium carbonate was used in Example 6-6.

[0104] The relation of electromotive force and carbon dioxideconcentration at room temperature at 30%, 50% and 70% humidity for thecarbon dioxide sensors of Examples 5-1 to 5-6 and 6-1 to 6-6 wereexamined like Examples 3-1 to 3-4.

[0105] The long-term stability test at high temperature ant humidity wasconducted for the carbon dioxide sensors of Examples 6-1 to 6-6.Specifically, the electromotive force was measured at room temperatureat the carbon dioxide concentration of 350 ppm and at 30% humidity, andit was kept in a constant temperature bath of 60° C. and 80% humidityfor 500 hours. After that, the electromotive force was again measured atroom temperature at the carbon dioxide concentration of 350 ppm and at30% humidity and the variation of electromotive force was determined.These results are shown in Table 3 and Table 4. TABLE 3 Variation ofMetal carbonate Sensitivity electromotive force (Non-fusion bonding(Number of reaction electron n) at 350 ppm CO₂ layer) 30% RH 50% RH 70%RH (30%→70% RH) Example 5-1 Na₂CO₃—BaCO₃ 2.1 2.0 2.1 12 mV Example 5-2Na₂CO₃ 1.8 1.9 1.9 13 mV Example 5-3 K₂CO₃—BaCO₃ 2.2 2.3 2.3 18 mVExample 5-4 K₂CO₃ 2.4 2.5 2.5 20 mV Example 5-5 Li₂CO₃—BaCO₃ 1.6 1.6 1.719 mV Example 5-6 Li₂CO₃ 1.6 1.6 1.5 29 mV

[0106] TABLE 4 Variation of electromotive force at 350 ppm CO₂ (mV)After stored at Metal carbonate Sensitivity high temperature (Non-fusion(Number of reaction electron n) Humidity and humidity bonding layer) 30%RH 50% RH 70% RH 30%→70% 30% RH Example 6-1 Na₂CO₃—BaCO₃ 1.7 1.6 1.6 1270 Example 6-2 Na₂CO₃ 1.6 1.6 1.7 21 100 Example 6-3 K₂CO₃—BaCO₃ 2.2 2.22.2 17 80 Example 6-4 K₂CO₃ 2.1 2.1 2.2 25 150 Example 6-5 Li₂CO₃—BaCO₃1.7 1.7 1.7 1 20 Example 6-6 Li₂CO₃ 1.7 1.8 1.8 7 90

[0107] As shown in Tables 3 and 4, similar to Examples 3-1 to 3-4 and4-1 to 4-6, Examples 5-1, 5-3, 5-5, 6-1, 6-3, and 6-5 including alkalimetal carbonate and barium carbonate have smaller variation ofelectromotive force by humidity. In addition, as shown in Table 4,Examples 6-1, 6-3 and 6-5 including alkali metal carbonate and bariumcarbonate have smaller variation of electromotive force after stored athigh temperature and humidity.

[0108] Comparing Table 3 with Table 4, the variation of electromotiveforce by humidity of Examples 6-3 and 6-5 is smaller than that ofExamples 5-3 and 5-5. Table 4 also reveals that Example 6-5 usinglithium carbonate as the first component can have very small variationof electromotive force as small as 1 mV.

[0109] More specifically, the carbon dioxide sensor shown in FIG. 3,which has the detection electrode 80 and the counter electrode 60 on thesame side of the electrolyte 70, is operatable at room temperature andreduce the influence of humidity when comprising the metal oxide layer81 and the metal carbonate layer 82 including the plural components,like the carbon dioxide sensor in which the detection electrode 80 isprovided on the opposite side of the counter electrode 60. It is morepreferable to form the carbon dioxide sensor at a temperature lower thanthe melting point of metal carbonate contained in the metal carbonatelayer 82 and to include lithium carbonate in the first component.

[0110] Incidentally, a carbon oxide sensor shown in FIG. 4 was formedand examined the properties like Examples 5-1 to 5-6 and 6-1 to 6-6, andit was confirmed that the same results were achieved.

Examples 7-1 and 7-2

[0111] The carbon oxide sensors were formed like Example 6-3 except thatcalcium carbonate was used instead of barium carbonate in Example 7-1and strontium carbonate was used instead of barium carbonate in Example7-2 as the second component. The complex carbonate of lithium carbonateand calcium carbonate was used in Example 7-1 and the complex carbonateof lithium carbonate and strontium carbonate was used in Example 7-2,and a metal carbonate layer 42 was formed at a temperature lower thanthe melting point of the metal carbonate. The relation of electromotiveforce and carbon dioxide concentration at room temperature at 30%, 50%and 70% humidity for the carbon dioxide sensors of Examples 7-1 and 7-2was examined and the long-term stability test at high temperature anthumidity was conducted, similar to Example 6-5. These results are shownin Table 5 together with the results of Examples 6-5 and 6-6. TABLE 5Variation of electromotive force at 350 ppm CO₂ (mV) After stored atMetal carbonate Sensitivity high temperature (Non-fusion (Number ofreaction electron n) Humidity and humidity bonding layer) 30% RH 50% RH70% RH 30%→70% 30% RH Example 6-5 Li₂CO₃—BaCO₃ 1.7 1.7 1.7 1 20 Example7-1 Li₂CO₃—CaCO₃ 1.9 1.9 2.0 4 40 Example 7-2 Li₂CO₃—SRCO₃ 1.9 1.8 1.8 650 Example 6-6 Li₂CO₃ 1.7 1.8 1.8 7 90

[0112] As shown in Table 5, like Example 6-5, Example 7-1 using calciumcarbonate instead of barium carbonate and Example 7-2 using strontiumcarbonate instead of barium carbonate could reduce the variation ofelectromotive force by humidity and after storage at high temperatureand humidity, compared to Example 6-6 using only lithium carbonate. Thatis, when using other alkali earth metal carbonate as the secondcomponent, we have found that it is operatable at room temperature andhas less influence of humidity.

Examples 8-1 to 8-8

[0113] The carbon dioxide sensors were formed like Example 6-1 exceptthat the second component was varied as shown in Table 6. That is, thecomplex carbonate of sodium carbonate and copper carbonate was used inExample 8-1, the complex carbonate of sodium carbonate and nickelcarbonate was used in Example 8-2, the complex carbonate of sodiumcarbonate and manganese carbonate was used in Example 8-3, the complexcarbonate of sodium carbonate and lead carbonate was used in Example8-4, the complex carbonate of sodium carbonate and zinc carbonate wasused in Example 8-5, the complex carbonate of sodium carbonate andindium carbonate was used in Example 8-6, the complex carbonate ofsodium carbonate and bismuth carbonate was used in Example 8-7 and thecomplex carbonate of sodium carbonate and cadmium carbonate was used inExample 8-8, and the metal carbonate layer 82 was formed at atemperature lower than the melting point of the metal carbonate. TABLE 6Variation of electromotive force at 350 ppm CO₂ (mV) After stored atMetal carbonate Sensitivity high temperature (Non-fusion (Number ofreaction electron n) Humidity and humidity bonding layer) 30% RH 50% RH70% RH 30%→70% 30% RH Example 6-1 Na₂CO₃—BaCO₃ 1.7 1.6 1.6 12 70 Example8-1 Na₂CO₃—CuCO₃ 2.4 2.3 2.5 16 70 Example 8-2 Na₂CO₃—NiCO₃ 2.3 2.1 2.119 80 Example 8-3 Na₂CO₃—MnCO₃ 2.1 2.1 2.2 14 60 Example 8-4Na₂CO₃—PbCO₃ 2.4 2.3 2.3 10 70 Example 8-5 Na₂CO₃—ZnCO₃ 2.1 2.0 2.0 1280 Example 8-6 Na₂CO₃—In₂(CO)₃ 2.3 2.2 2.2 15 70 Example 8-7Na₂CO₃—Bi₂(CO)₃ 2.4 2.3 2.1 18 80 Example 8-8 Na₂CO₃—CdCO₃ 2.5 2.4 2.413 90 Example 6-2 Na₂CO₃ 1.6 1.6 1.7 21 100

[0114] The relation of electromotive force and carbon dioxideconcentration at room temperature at 30%, 50% and 70% humidity for thecarbon dioxide sensors of Examples 8-1 to 8-8 was examined and thelong-term stability test at high temperature ant humidity was conducted,similar to Example 6-1. These results are shown in Table 6 together withthe results of Examples 6-1 and 6-2.

[0115] As shown in Table 6, like Example 6-1, Example 8-1 to 8-8 usingtransition metal carbonate, lead carbonate, zinc carbonate, indiumcarbonate, bismuth carbonate or cadmium carbonate instead of bariumcould reduce the variation of electromotive force by humidity and afterstorage at high temperature and humidity, compared to Example 6-2 usingonly sodium carbonate. That is, when using the transition metalcarbonate, lead carbonate, zinc carbonate, indium carbonate, bismuthcarbonate or cadmium carbonate as the second component, we have foundthat it is operatable at room temperature and has less influence ofhumidity.

Examples 9-1 and 9-2

[0116] The carbon dioxide sensors were formed like Example 6-1 exceptthat rubidium carbonate was used instead of sodium carbonate in Example9-1 and cesium carbonate was used instead of sodium carbonate in Example9-2 as the first component. That is, the complex carbonate of rubidiumcarbonate and barium carbonate was used in Example 9-1 and the complexcarbonate of cesium carbonate and barium carbonate was used in Example9-2, and the metal carbonate layer 42 was formed at a temperature lowerthan the melting point of the metal carbonate. The relation ofelectromotive force and carbon dioxide concentration at room temperatureat 30%, 50% and 70% humidity for the carbon dioxide sensors of Examples9-1 and 9-2 was examined and the long-term stability test at hightemperature and humidity was conducted, similar to Example 6-1. Theresults are shown in Table 7. TABLE 7 Variation of electromotive forceat 350 ppm CO₂ (mV) After stored at Metal carbonate Sensitivity hightemperature (Non-fusion (Number of reaction electron n) Humidity andhumidity bonding layer) 30% RH 50% RH 70% RH 30%→70% 30% RH Example 9-1Rb₂CO₃—BaCO₃ 1.4 1.5 1.6 25 80 Example 9-2 Cs₂CO₃—BaCO₃ 1.5 1.6 1.5 2370

[0117] As shown in Table 7, Examples 9-1 and 9-2 using rubidiumcarbonate or cesium carbonate as the first component could reduce theinfluence of humidity at the same level as Example 6-2 using only sodiumcarbonate. Although it is not shown in Table 7, it can be consideredthat since rubidium carbonate and cesium carbonate have deliquescence,they are affected by humidity when using alone and the sufficientproperties cannot be achieved. That is, when the carbon dioxide sensorcomprises the second component when using other alkali metal carbonateas the first component, we have found that it is operatable at roomtemperature and has less influence of humidity.

Example 10-1

[0118] A carbon dioxide sensor shown in FIG. 2 was formed like Example3-2 except that the complex carbonate of lithium carbonate and bariumcarbonate, and the composite oxide of indium and tin was mixed at 1:1 inmass ratio to prepare paste and applied to the electrolyte 70 and heatedat 500° C. for 30 minutes to form the detection electrode 50. Anothercarbon dioxide sensor was also formed like Example 10-1 except thatlithium carbonate and the composite oxide of indium and tin was mixed at1:1 in mass ratio to form the detection electrode as Comparative Example10-1. Comparative Example 10-1 is the same as Comparative Example 1.

[0119] The relation of electromotive force and carbon dioxideconcentration at room temperature at 30%, 50% and 70% humidity for thecarbon dioxide sensors of Example 10-1 and Comparative Example 10-1 wasexamined like Example 3-2, and the long-term stability test at hightemperature ant humidity was conducted, similar to Example 6-3. Theresults are shown in Table 8. TABLE 8 Variation of electromotive forceat 350 ppm CO₂ (mV) After stored at Metal carbonate Sensitivity hightemperature (Non-fusion (Number of reaction electron n) Humidity andhumidity bonding layer) 30% RH 50% RH 70% RH 30%→70% 30% RH Example 10-1Li₂CO₃—BaCO₃ 1.9 2.0 2.0 5 30 Example 10-2 Li₂CO₃ 1.7 1.8 1.8 18 100

[0120] As shown in Table 8, Example 10-1 diffusing the complex carbonateof lithium carbonate and barium carbonate could reduce the variation ofelectromotive force by humidity and after stored at high temperature andhumidity like Examples 3-2, 4-3, 5-3 and 6-3, compared to ComparativeExample 10-1 using lithium carbonate alone. That is, it is fount thatthe carbon dioxide sensor forming the detection electrode 50 bydiffusing metal carbonate and metal oxide as shown in FIG. 2 isoperatable at room temperature and has less influence of humidity ifmetal carbonate including the plural components was contained.

[0121] Although the invention has been described by the foregoingembodiments and examples, the invention is not limited to theembodiments and the examples but can be variously modified. For example,in the first embodiment and examples, the detection electrodes 10 and 80comprise the metal oxide layers 11 and 81 and the metal carbonate layers12 and 82, or the detection electrode 50 is formed by the layerincluding metal oxide and metal carbonate of the plural components.However, it may further include other components. The metal oxide layers11 and 81 may contain other substances in addition to theabove-described metal oxides and the metal carbonate layers 12 and 82may contain other substances other than the above-described metalcarbonates and metal hydrogen carbonates. Further, the detectionelectrodes 50 and 80 may contain other substances other than theabove-described metal oxides, metal carbonates and metal hydrogencarbonate.

[0122] As described above, according to the first carbon dioxide sensorof the invention, the metal oxide layer and the metal carbonate layerplaced between the metal oxide layer and the electrolyte are included.As a result, it is operative at room temperature and has less influenceof humidity, and furthermore, the carbon dioxide sensing property can beimproved. Therefore, the carbon dioxide concentration can be easilydetected with high accuracy.

[0123] Specifically, according to the carbon dioxide sensor in which themetal carbonate layer contains lithium carbonate and the carbon dioxidesensor in which the metal carbonate layer is formed at a temperaturelower than the melting point of containing metal carbonate, theinfluence of humidity can be reduced and the carbon dioxide sensingproperty can be improved.

[0124] According to the second carbon dioxide sensor of the invention,the detection electrode comprises the metal oxide and the metalcarbonate including plural components. As a result, it is operative atroom temperature and has less influence of humidity and therefore, thecarbon dioxide concentration can be easily detected with high accuracy.

[0125] Specifically, according to the carbon dioxide sensor in which thedetection electrode include the fist component of alkali metal carbonateand the second component of at least one element selected from the groupconsisting of alkali earth metal carbonate, transition metal carbonate,zinc carbonate, cadmium carbonate, indium carbonate, lead carbonate andbismuth carbonate, and further the first component includes lithiumcarbonate and the second component includes at least one elementselected from the group consisting of barium carbonate, potassiumcarbonate and strontium carbonate, the influence of humidity can bereduced.

[0126] Further, according to the carbon dioxide sensor in which thedetection electrode comprises the metal oxide layer and the metalcarbonate layer placed between the metal oxide layer and theelectrolyte, the influence of humidity can be reduced and the carbondioxide sensing property can be improved.

[0127] Furthermore, according to the carbon dioxide sensor in which themetal carbonate layer is formed at a temperature lower than a meltingpoint of containing metal carbonate, the higher effects can be achieved.

What is claimed is:
 1. A carbon dioxide sensor provided with a detection electrode and a counter electrode on an electrolyte, wherein the detection electrode comprises a metal oxide layer including a metal oxide and a metal carbonate layer including a metal carbonate placed between the metal oxide layer and the electrolyte.
 2. A carbon dioxide sensor according to claim 1, wherein the metal carbonate layer includes lithium carbonate.
 3. A carbon dioxide sensor according to claim 1, wherein the metal carbonate layer is formed at a temperature lower than a melting point of containing metal carbonate.
 4. A carbon dioxide sensor according to claim 1, wherein the metal oxide layer comprises at least one element selected from the group consisting of tin oxide, indium oxide, cobalt oxide, tungsitc oxide, zinc oxide, lead oxide, copper oxide, iron oxide, nickel oxide, chromium oxide, cadmium oxide, bismuth oxide, manganese oxide, yttrium oxide, antimony oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, silver oxide, lithium oxide, sodium oxide, potassium oxide, rubidium oxide, magnesium oxide, calcium oxide, strontium oxide and barium oxide.
 5. A carbon dioxide sensor according to claim 4, wherein the metal oxide layer contains a composite oxide including tin and indium.
 6. A carbon dioxide sensor according to claim 1, wherein the electrolyte includes a metal ion conductor.
 7. A carbon dioxide sensor provided with a detection electrode and a counter electrode on an electrolyte, wherein the detection electrode comprises a metal oxide and a metal carbonate including plural components.
 8. A carbon dioxide sensor according to claim 7, wherein the metal carbonate comprises a first component of alkali metal carbonate and a second component of at least one element selected from the group consisting of alkali earth metal carbonate, transition metal carbonate, zinc carbonate, cadmium carbonate, indium carbonate, lead carbonate and bismuth carbonate.
 9. A carbon dioxide sensor according to claim 8, wherein the first component includes lithium carbonate.
 10. A carbon dioxide sensor according to claim 8, wherein the second component includes at least one element selected from the group consisting of barium carbonate, calcium carbonate and strontium carbonate.
 11. A carbon dioxide sensor according to claim 7, wherein the detection electrode comprises a metal oxide layer including the metal oxide and a metal carbonate layer including the metal carbonate placed between the metal oxide layer and the electrolyte.
 12. A carbon dioxide sensor according to claim 11, wherein the metal carbonate layer is formed at a temperature lower than a melting point of containing metal carbonate.
 13. A carbon dioxide sensor according to claim 7, wherein the metal oxide comprises at least one element selected from the group consisting of tin oxide, indium oxide, cobalt oxide, tungsitc oxide, zinc oxide, lead oxide, copper oxide, iron oxide, nickel oxide, chromium oxide, cadmium oxide, bismuth oxide, manganese oxide, yttrium oxide, antimony oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, silver oxide, lithium oxide, sodium oxide, potassium oxide, rubidium oxide, magnesium oxide, calcium oxide, strontium oxide and barium oxide.
 14. A carbon oxide sensor according to claim 13, wherein the metal oxide layer contains a composite oxide including tin and indium.
 15. A carbon oxide sensor according to claim 7, wherein the electrolyte includes a metal ion conductor.
 16. A carbon oxide sensor according to claim 7, wherein the detection electrode and the counter electrode are provided on the same side of the electrolyte. 